This invention relates to producing tubes from hollow workpieces, and more in particular to a method and apparatus for producing tubes from mills which have features of prior art mills known as a McKay rocker mill or a FHR reciprocating crosshead type rocker mill. The mill used in practicing the present invention also has features disclosed in U.S. Pat. No. 3,487,675.
Other patents relating to the production of tubular products are disclosed in U.S. Pat. Nos. 4,090,386 and 4,233,834.
U.S. Pat. No. 4,090,386 relates to a method of producing zircaloy tubes of small internal diameter where it is desirable to have high rates of reduction in the wall thickness. The disclosure is directed to zircaloy tubes having close tolerances in inside and outside diameters and from the standpoint of ovality. Further, the rocker mechanism produces tubes with a fine grain structure, improved hydride orientation and higher ratios of strength to ductility. The rocker mechanism permits the workpiece to be turned around its axis a predetermined number of degrees for each time that the workpiece is advanced one step into the forming zone. The mandrel, employed in the production of the tubing, is securely locked within the cross-head of the device.
U.S. Pat. No. 4,233,834 is directed to a method and apparatus for producing zircaloy tubes and the tubing produced by the apparatus in which the spiral formation of the wall-thickness eccentricity is controlled. This is done by controlling the angle at which the metal working forces are exerted so as to minimize the turning moment or torque effect which results in producing the spiral formation of the tubing.
An object of the present invention is to provide improved methods for producing high-quality metal alloy tubes. A further object is to provide for the above in a manner to permit a reduction in the time required to form finished tubes from tubes of greater wall thickness than the finished tubes. A further object is to provide for the above in a manner which overcomes difficulties encountered in the past and which permits high rates of reduction of wall thickness in forming such tubes. This difficulty is encountered when working with metals which do not lubricate well together.
The present invention is particularly directed toward producing metal alloy, most preferably, titanium or zirconium alloy, tubes of small internal diameter from cylindrical workpieces where it is desirable to have high rates of reduction in the wall thickness. It has been found that the invention permits high output rate with increased reduction rates and with less tool cost as represented by avoidance of breakage of mandrels. In the production of metal alloy tubes, such as, for example, titanium or zirconium alloy tubes, cylindrical mandrels may encounter failure by tensile fracture or reduction in diameter during rocking when manufacturing tubes of small O.D., i.e., on the order of 0.250". The problem encountered in the production of tubes of this type is that stress in the mandrel is a combination of compressive stresses in the rolling operation and tensile stresses set up in the mandrel, which is restrained at one end, and held by friction of the reduction cone at the other.
The present invention obviates and/or reduces the problems by utilizing a longer mandrel which is restrained at the rocker crosshead by friction instead of a positive locking force. This arrangement permits the mandrel to slide in the crosshead, upon the application of the proper friction force. Accordingly, the high tensile forces which would otherwise cause mandrel failure are not produced and mandrel life is significantly increased. Other cost savings in equipment downtime, labor time for replacement of mandrels, etc., are also realized.